In general, mobile displays can be classified into displays for mobiles, and displays for TVs and monitors. For such display panels, it is an important issue to improve outdoor visibility and impact strength so as to correspond to increase in panel size. In this regard, optical adhesive films serve to improve outdoor visibility and impact resistance.
A display panel includes a glass window layer as an outermost layer, an air layer placed under the window layer, and an ITO glass placed under the air layer. The air layer is filled with a transparent material having a similar index of refraction to that of the window glass to improve outdoor visibility.
Currently, materials used as optical adhesives are classified into liquid type and film type materials. Despite the use convenience, film type materials have difficulty in application to various structures and removal of bubbles created upon attachment to a substrate. Moreover, since the film type material has high internal cohesion and thus does not allow easy removal, the film type material has undesirable reworkability and is produced through a complicated process causing increase in manufacturing costs.
Accordingly, development of inexpensive liquid phase materials allowing easy removal of bubbles is a recent trend in the art. Such a liquid phase material is formed into a film through photocuring. A liquid type adhesive is formed into a film through a process of filling a matrix with the liquid type adhesive, followed by photocuring of the liquid type adhesive. Although such a liquid type adhesive allows easy removal of bubbles, there is a problem of high contraction rate upon photocuring. In addition, a typical liquid phase adhesive composition has a high acid value of less than pH 4, thereby causing substrate corrosion.
Further, with the recent trend of increasing the size of mobile displays, a photocurable material is often separated from an upper material, such as glass, polycarbonate or polymethyl methacrylate, due to deterioration in adhesion therebetween by contraction upon photocuring. Particularly, isoprene or butadiene resins used as rubber materials applicable to displays have a limitation in enhancement of adhesion after photocuring. Moreover, with increasing sizes of display products, there is a need for a material having lower viscosity than that of typical materials in order to allow more efficient dispensing of liquid materials.